DESCRIPTION: The investigators have determined that the LTC-IC assays differ significantly between C57/BL-6, DBA-2 and BALB-c mice. Loci termed Scfr-1 and Scfr-2 are proposed and investigators will focus on positionally cloning these genes and their human homologs. They will do this by generating mouse backcrosses to localize the Scfr genes within 1 Mb fragments. These regions will then be used to evaluate overlapping arrays of yeast and bacterial artificial chromosomes. Definition of the Scfr genes will be based on several criteria such as cDNA selection and expression polymorphisms. In addition, the investigators will analyze which cells in the stem cell hierarchy and at which stages of development strain specific differences are seen and whether or not these are also identified in localized peripheral blood samples.